Liver transplantation is the primary treatment for patients with end-stage liver disease, with over 6,000 liver transplants performed each year in the U.S. There are almost 16,000 people currently on waiting lists and this number is expected to increase despite efforts to expand the donor pool. The epidemic in obesity and diabetes has contributed to the problem by increasing both the number of liver transplants needed and donor organ nonuse. Fatty liver disease (hepatic steatosis) is the most prevalent adverse condition in liver grafts, wherein the fat can be classified as macrovesicular or microvesicular. Macrovesicular steatosis is considered a primary risk factor in transplanted livers for both primary non-function and dysfunction and can therefore result in organ nonuse. Historically, livers judged to have a fat content higher than 30% were rejected for transplantation. However, due to the dire need to expand the donor pool, livers that are considered moderately fatty (30-60%) are starting to be used. To effectively use these livers, an accurate quantification of fat content will be needed to facilitate successful matching of livers with recipients. At present, the gold standard for hepatic steatosis evaluation is biopsy and histological assessment. However, this method is invasive, relies on highly trained personnel, and produces subjective results. While state-of-the-art imaging modalities have emerged to determine fat content, these methods are costly, time intensive, and unable to differentiate between macro- and micro-vesicular steatosis. To fill this gap in available diagnostic tools, ultrasound tissue characterization methods have promise. Such methods utilize the interactions between ultrasound waves and tissue to identify mechanical tissue properties. While such approaches have been used to characterize liver tissue in terms of total fat content, none have focused on distinguishing micro- and macro-vesicular components. Here, we propose to develop, implement, and test such a method. Our approach comprises a combination of well-developed techniques: measurements of sound speed and the nonlinear ratio B/A predict total fat content, while measurements of attenuation as a function of frequency will enable estimates of the relative composition of micro- and macro-vesicular fat. The first and second proposal aims seek to build an acoustic caliper device operating in transmission mode, implement advanced methods for measuring and interpreting B/A, sound speed, and attenuation, and refine these functionalities based on measurements from lipid-emulsion phantoms and fatty pig livers. The third aim seeks to collect in vivo and ex vivo acoustic measurements from steatotic pig livers as well as human donor livers rejected for transplantation. Results of these measurements will be evaluated to demonstrate proof-of-principle by comparison with gold-standard metrics currently used for transplants. We have assembled a team of experts in the areas of acoustics, tissue evaluation, and liver transplantation at the University of Washington, as well as partners in the broader transplant community (LifeCenter Northwest) to accomplish proposal aims and address a major clinical need for liver transplantation.